456 



DISCS AND NUCLEI IN A MUSCULAR FIBRE. 



stretched and stained with logwood, the central part of the dim disc appears lighter in colour 

 than the two ends of the same disc. This lias been described as a separate disc, and is called 

 the median disc of Hensen (fig. 302, 4, c).] 



[In an unaltered fibre, the dim broad stripe may appear homogeneous, but after a time it 

 cleaves throughout its entire extent in the long axis of the fibre into a number of prismatic 

 elements or fibrils, the sarcous elements of Bowman (fig. 302). These at first are prismatic, but 

 as ".they solidify they shrink and seem to squeeze out of them a fluid, becoming at the same time 

 more constricted in the centre. This separation into bundles of fibrils with an interstitial matter 

 gives rise to the appearance seen on transverse section of a frozen muscle, and known as Cohn- 

 heim's areas (fig. 302, 2, c). In all probability the cleavage also extends through the lateral 

 discs, and thus fibrils are formed by longitudinal cleavage of the fibre.] 



[Muscles of Arthropoda. Engelmann showed that the muscles of these animals have a large 

 number of discs. In a muscle of an animal killed by being plunged into alcohol, according to 

 the position of the lens of the microscope, one sees : 



1. The broad dim disc, composed of two darker lateral portions or discs, and a lighter disc 

 that of Hensen, between them. In fig. 304 the whole disc is marked Q, and Hensen's disc is 

 distinguished as h. 



2. On both sides of this is a small, clear, slightly refractive stripe, J, corresponding to one of 

 Engelmann's isotropous stripes. 



3. On both sides there follows symmetrically a dark strongly refractive stripe, N, corre- 

 sponding to Engelmann's accessory stripe and Flbgel's granular layer. 



4. Then on both sides there is a clear, feebly refractive disc, E. 



5. Beyond E is a small, dark, highly refractive stripe, Z usually the darkest corresponding 

 to the Amici-Krause line.] 



[From Z, the stripes are repeated in the inverse order to Q, then in the same order to Z, and 

 so on. This is the appearance with a low position of the lens. Many muscles do not show all 

 these stripes, thus h is often absent.] 



[If the lens of the microscope be raised, to get a more superficial view of the fibre, the 

 distribution of the light is reversed (fig. 304, II), as all strongly refractive sections become 



light, and all feebly refrac- 

 2? ^sszssnssssssssA Z tive appear darker, while 

 with a deep position of the 

 lens, the reverse is the 

 case. ] 



[Experiment shows that 

 the dim disc rapidly swells 

 up in dilute acids, and also 

 that the dim discs (Q), the 

 accessory discs (N), and 

 the Amici-Krause line (K), 

 stain more deeply with log- 

 wood than the other discs, 

 and h less than the rest of 



Q.] 



[If a muscle which has 

 been some time in alcohol 

 be examined as to its longi- 

 tudinal striation, it will be 

 seen to consist of rods with 

 light intervals between 

 them (fig. 305). The rods 

 are thicker at their ends, and thinner and lighter at their middle. Rollett regards the clear 

 intervals between these rods as consisting of sarcoplasma, a body closely related to protoplasm, 

 and the rods as bundles of fibrillar or "muscle columns."] 



[If a muscle be acted upon by certain acids the relative appearance of the muscle-columns and 

 the sarcoplasma is altered ; and the latter may appear in these and in gold preparations as a 

 plexus of fibrils with regular longitudinal and transverse meshes (Mclland, Marshall, fig. 306).] 

 [Muscle Bods. Schafer describes the appearance differently: Double rows of granules are 

 seen lying in or at the boundaries of the light streaks (discs), and very fine longitudinal lines 

 may be detected running through the dark streak (dim disc) and uniting the minute granules. 

 These fine lines, with their enlarged extremities, are " muscle rods." They are most conspicuous 

 in insects. During the contraction of a living muscular fibre, Schafer describes the "reversal 

 of the stripes" ( 297) as follows : "When the fibres contract, the light stripes are seen, as 

 the fibre snortens and thickens, to become dark, an apparent reversal being thereby produced 

 in the strise. This reversal is due to the enlargement of the rows of dark dots and the formation 

 by their juxtaposition and blending of dark discs, whilst the muscular substances between these 

 discs has by contrast a bright appearance." With polarised light in a living muscular fibre, 



Fig. 304. Fig. 305. 



Fig. 304. Insect's muscle; I, with a high position of the lens, 

 and II, with a deeper position. Fig. 305. Muscular fibre of 

 Carabus cancellatus. 



